Proposed work for the 1979-80 grant year involves extension of the studies carried out in the current year. Two distinct areas of research are involved: 1) The kinetics of interaction between adrenal steroid 11beta-hydroxylase and an activator present in adrenal cytosol. The proposed studies in this area involve: a) determination of kinetic changes of interaction between these components and stage of pregnancy; b) determination of the equilibrium scheme of interaction between enzyme, substrate, and activator; c) elucidation of physicochemical properties of activator. 2) Equilibrium interactions between progesterone and placental progesterone receptors. The proposed studies in this area include: a) elucidation of kinetic interaction between receptor, ligand, and a diffusible inhibitor present in placental cytosol; b) validation of an exchange assay for placental nuclear receptors; c) study characteristics of progesterone translocation to the nucleus and interaction between corticosteroids.